A heating system by use of a heater core for a vehicle is known in the art, according to which waste heat of an internal combustion engine is utilized. For example, as disclosed in Japanese Patent No. 5,042,816, in a hybrid vehicle having the heating system by use of the heater core, an internal combustion engine is operated in a first operating mode or in a second operating mode. In the first operating mode, the engine is operated at a constant engine rotational speed at which a value for BSFC (Brake Specific Fuel Consumption) is minimized when temperature of engine cooling water is lower than a predetermined value. In the second operating mode, the engine is operated to generate electric power, which satisfies only electric power to be consumed by accessory devices of the vehicle.
In the first operating mode of the vehicle control system of the above prior art, the engine operation is controlled in such a way that the value of the BSFC is minimized. Energy efficiency of the engine itself can be maximized when the engine is operated in the engine operating mode for minimizing the BSFC. However, it is not always true that the energy efficiency can be also maximized, when the energy efficiency is evaluated as a whole system, because there exist some energy losses in the whole system of a vehicle control, such as, a motor-generator loss, a battery loss and so on.